III-V group nitride compound semiconductor having a wide bandgap of about 1.9 eV (InN) to about 6.2 eV (AlN) has been widely used for a light emitting diode (LED), a laser diode (LD), a UV detector, etc., generating light with a wavelength in a range of about UV region and visible light region. Aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), etc. have been considered as a useful material for optical and electrical application, since these materials may have continuous solid solution such as InGaN, InAlN, AlGaN, etc., and it is possible to adjust wavelength of light generated by a light generating device by changing a ratio of compound of quantum well layer of the light generating device.
A band gap of the nitride compound in a Wurtzite phase is in a range of about 1.9 eV to about 6.2 eV In detail, a-InN has a band gap of about 1.6 eV, a-GaN has a band gap of about 3.4 eV, and a-AlN has a band gap of about 6.2 eV. Furthermore, comparing conventional ZnS, InP and GaAs, nitride compound has relatively high thermal conductivity, and relatively high melting point so that the nitride compound is stable in high temperature. Additionally, nitride compound has good mechanical, physical and chemical characteristics such as corrosion resistance, thermal endurance, etc., so that nitride compound is noted not only for optical device but also for high temperature, high voltage, high power, high frequency electronic device.
Electronic devices based on GaN has been rapidly developed as a technique for growing thin film of GaN has been developed in 1980s. Akasaki et al. obtained good GaN thin film without cracks, using AlN buffer layer on sapphire (Al2O3) substrate through metalorganic chemical vapor deposition (MOCVD) method. In order to obtain good GaN crystal thin film, it is necessary to use substrate having a lattice constant and a coefficient of thermal expansion which are the same as that of GaN. That is, in order to reduce defects in the GaN thin film layer, a homo-epitaxy method using bulk GaN as a substrate is much more advantageous than a hetero-epitaxy method.
Nevertheless, GaN has not been used as a substrate because of high melting point and partial pressure of nitrogen. The melting point of GaN is about 2800 K, and the partial pressure of nitride is about 103 atm at a temperature of about 1200° C. Therefore, the homo-epitaxy method using bulk GnN is not widely used because of critical condition of the high melting point and the partial pressure of nitrogen.
As a result, the GaN thin film is grown on a different substrate, for example such as (0001)sapphire, (0001)SiC, etc. The (0001)sapphire substrate has about 13% lattice defect with GaN, and the (0001)SiC substrate has about 4% lattice defect with GaN. Therefore, the (0001)SiC substrate is more effective. However, the (0001)SiC substrate is less productive than the (0001)sapphire substrate, so that the (0001)SiC substrate is ten times expensive than the (0001)sapphire substrate. Therefore, most research groups use the (0001)sapphire substrate for growing GaN.
However, many defects may be generated at a boundary region between the (0001)sapphire substrate and the GaN thin film due to lattice defect of the 13%, and the 13% difference in the coefficient of thermal expansion between the (0001)sapphire substrate and the GaN thin film during a process of growing an epytoxic layer. The above defects prevent electrons from moving or the above defects are operated as a non-radiative recombination center to deteriorate electrical and optical characteristics of GaN. Therefore, spinel (MgAl2O4), GaAs, LiGaO2, MgO, etc., which have relatively small amount of lattice defects, have been researched as a material for the substrate, but these materials also are not so much fruitful. Furthermore, a sapphire substrate of M-plane(1010), R-plane(1102), A-plane(1120), etc., has been researched in place of a substrate of C-plane(0001), but these researches are not so much fruitful, also.
Therefore, other researches for minimizing defects such as a threading dislocation, which is generated during growing GaN on a sapphire substrate, have been performed. The representative example for minimizing defects is a buffer layer between the sapphire substrate and the GaN.
In 1983, Yoshida et al. use sapphire coated with AlN for the first time, Akasaki group reduces crack by depositing AlN buffer layer at a low temperature, and nowadays a low temperature GaN buffer layer are frequently used. Now, the low temperature GaN buffer layer is considered to be essential technology for growing GaN through a MOCVD method. The amorphous state or polycrystal low temperature buffer layer having about 20˜40 nm thickness reduces deterioration of crystal structure, which is induced by lattice unconformity to grow GaN layer of high quality.
The low temperature AlN or GaN buffer layer make it possible to grow GaN layer on sapphire substrate. Furthermore, Amano et al. grow a GaN layer containing magnesium (Mg) through a low-energy electron beam irradiation (LEEBI) method to form a p-type GaN layer. As a result, a light generating device based on GaN is rapidly developed. However, the above LEEBI method has some weak point. That is, the electron beam has limitation for penetrating a GaN wafer. Therefore, only a surface of GaN wafer becomes the p-type.
Nakamura et al. obtained p-type GaN layer, of which resistivity is about 2 Ω·cm, under the condition of 700° C. by thermal annealing treatment. Further more Nakamura et al. diagnoses the effect of thermal annealing treatment and the effect of a hole compensation of p-type GaN by adjusting gas during thermal annealing treatment. According to the diagnoses, Mg—H complexes increases the resistance of GaN. Therefore, by removing hydrogen in Mg—H complexes in thermal annealing treatment using N2 gas, the resistance can be lowered. Using the above, Nichia corporation commercialized a blue LED.
However, when the c-plane(0001), which is polar, is used as the growing plane of GaN, piezoelectric field is generated due to piezoelectric effect, so that hole and electron pair is separated by the piezoelectric field to reduce probability of combination of hole and electron. Therefore, efficiency is lowered.